Email slow to download on some address not others

[#1]

I have a number of email addresses with PlusNet all using POP to download. All set up on Thunderbird and with the same paramaters. One in particular is very slow to download once connected, sometimes never compleats the download of new message headers, sometimes only some. Once headers are downloaded then often can't download the body of message.

Other email addresses are OK. This is all using a number of desktop computers connected via ethernet cable. Any helpfull  suggestions ?

[#2]

This seems somewhat variable, occasional 30 or more email headings download quickly then trying to download content of a few can simply stop. Sometimes it can all be quick, but often on one specific address it can take a day or two.

It seems more likely to be PlusNet server than any of my computers.